A display device using a liquid crystal compound (in this application a liquid crystal compound is used as a generic term for a compound having the nematic phase, and a compound having no liquid crystal phase but useful as a component of a liquid crystal composition) is widely used for displays of watches, calculators, word processors and so forth. This display device utilizes the refractive index anisotropy and the dielectric anisotropy of the liquid crystal compound.
A liquid crystal display device typified by a liquid crystal display panel, a liquid crystal display module and so forth, utilizes optical anisotropy, dielectric anisotropy and so forth, which are possessed by a liquid crystal compound. As operating modes of this liquid crystal display device, a variety of modes are known, such as PC (phase change), TN (twisted nematic), STN (super twisted nematic), BTN (bistable twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS (in-plane switching), VA (vertical alignment) and PSA (Polymer sustained alignment) modes.
The liquid crystal display device comprises a liquid crystal composition having suitable physical properties. To improve the characteristics of the liquid crystal display device, it is desirable that the liquid crystal composition has suitable physical properties. It is required for liquid crystal compounds, which are components of the liquid crystal composition, to have characteristics shown in items (1) to (7) below. That is to say:    (1) being chemically stable and physically stable,    (2) having a high clearing point (transition temperature on a liquid crystal phase—an isotropic phase),    (3) having a low minimum temperature of a liquid crystal phase (a nematic phase, a smectic phase and so forth), especially of the nematic phase,    (4) having a small viscosity,    (5) having a suitable optical anisotropy,    (6) having a suitable dielectric anisotropy, and    (7) having an excellent compatibility with other liquid crystal compounds.
A voltage holding ratio can be increased by using a composition containing a chemically and physically stable liquid crystal compound as described in item (1), for a display device. The temperature range of a nematic phase is wide in a composition that contains a liquid crystal compound having a high clearing point or a low minimum temperature of a liquid crystal phase as described in items (2) and (3), and thus the device is usable in a wide temperature range.
Furthermore, when a composition containing a compound having a small viscosity as described in item (4) and a compound having a large elastic constant K33 as described in item (7) is used for a display device, the response speed can be improved. In the case of a display device using a composition that contains a compound having a suitable optical anisotropy as described in item (5), an improvement of the contrast in a display device can be expected. From a small to a large optical anisotropy is required according to the design of a device. Recently, a method for improving the response speed by means of a smaller cell thickness has been investigated, whereby a liquid crystal composition having a large optical anisotropy has also been required.
Moreover, when a liquid crystal compound has a large negative dielectric anisotropy, the threshold voltage of the liquid crystal composition containing this compound can be decreased. Hence, the driving voltage of a display device can be decreased and the electric power consumption can also be decreased in the case of a display device using a composition containing a compound that has a suitable dielectric anisotropy as described in item (6).
The liquid crystal compound is generally used as a composition prepared by mixing it with many other liquid crystal compounds in order to exhibit characteristics that cannot be attained with a single compound. Accordingly, it is desirable that a liquid crystal compound used for a display device has an excellent compatibility with other liquid crystal compounds and so forth, as described in item (7). Because the display device may also be used in a wide temperature range including a lower temperature than the freezing point, the compound that exhibits an excellent compatibility even in a low temperature range may be desirable.
Compounds having a cyclohexane ring and a benzene ring as described below are known as a liquid crystal compound used for this purpose. For example, the compounds (A) and (B) are disclosed in patent document No. 1, the compound (C) is disclosed in patent document No. 2, and the compound (D) is disclosed in patent document No. 3. No compounds disclosed in any of the documents have a sufficiently high clearing point. Thus, the temperature range of the nematic phase of the compositions that contain these compounds is not sufficiently wide. Further, the compositions that contain these compounds have a disadvantage that they are not suitable for a device with a large cell thickness for the high-speed response, because the optical anisotropy is not sufficiently large.
